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My aim is to add more structure to my riding over the winter to improve XC/ Marathon riding. I have just picked up a Taxc Booster as I run 1x10 gearing and easily run out of resistance on my previous Turbo. I was considering signing up to Trainerroad to help with getting the structure in to the Turbo sessions, especially since getting the new Trainerroad compatible trainer....

However I got thinking that the software is essentially using the relationship between speed and resistance setting on the trainer to estimate power. Whether or not the power number means anything is irrelevant as far as I'm concerned as you would do a FTP tests then do the sessions as some percentage of the FTP "power" depending on your goals.

Now since the trainer resistance and speed relationship to estimate "power" is linear why not just train by speed? So long as the resistance does not vary from the FTP test (for example resistance setting or trainer/ wheel contact force), is there any issue with training to a percentage of your FTP speed?

For example if you do and FTP test on resistance setting 4 and find you have an FTP of 25mph can you use speed do an endurance, Z2 session (56%-75% of FTP) by riding in the 14-18mph range on resistance setting 4?

The main problem would be the resistance curve. Most trainers resistance is not linear, manufacturers try to mimic the resistance you would see on the road which is more exponential. TrainerRoad has mapped out resistance curves for various trainers using a power meter to make their software more accurate. I think in the absence of a power meter (real or virtual) you would do better training by heart rate.

Good point on the resistance/power/ speed relationship perhaps not being linear on all trainers. I have checked with Tacx and the Booster is, so inferring power directly from speed is linear and involves no fancy calculations to train by "power". So in effect knowing speed I have a virtual power meter.

Why is estimated or virtual power useless?

Of course the numbers may be meaningless but as far as I can tell if your FTP was 100 or 1000 is doesn't matter as you train to percentages of that FTP.

The issue is that an estimate of power cannot be calibrated accurately or the method of calibration unable to be quantified.

For example: a SRM can be calibrated with a known weight and has an actual way of measuring torque (strain gauges).

Trainerroad and similar base it of variables which can fluctuate grossly and you'd be unaware... unless you have a power meter to calibrate it.

The whole benefit to power is that it is an accurate measure of work. Having a system which is not accurate and you're better off deferring to measure of effort which would yield a more accurate representation - in this case HR and/or RPE.

"Physiology is all just propaganda and lies... all waiting to be disproven by the next study.""I'm not a real doctor; But I am a real worm; I am an actual worm." - TMBG